Browsing by Author "Ayalew, Dereje (Professor)"
Now showing 1 - 13 of 13
Results Per Page
Sort Options
Item Compositional Variation of Erta’ale Basaltic Eruption with Time(Addis Ababa Universty, 2017-06) Lemma, Edelawit; Ayalew, Dereje (Professor)Erta’Ale lava lake is located at the center of the Erta’Ale volcanic range in the northern afar depression. Erta’Ale is known by its two active lava lake situated at northern and central part of the caldera. The thesis generally considered the compositional variation of the Erta’Ale lava through time. Field investigation, petrographic and whole rock geochemical studies have been applied to meet the objectives. The lava of Erta’Ale volcano has MgO content ranging from (4.56-6.36 wt%) and SiO2 content (47-52 wt%) which decrease with increasing of MgO. The Al2O3 and CaO contents slightly increase with decreasing MgO down to ~6 wt%, and then decrease with decreasing of MgO a general positive trend for Fe2O3 and TiO2. The variations diagram of the Erta’Ale lava show fractionation of mineral phase. In most of the Erta’Ale lava (Ce/Pb > 20) crustal contamination is not major process. However there are some samples which have low value of Ce/Pb ratio (<20) because of their high value of Pb . The Erta’Ale lava is characterized by low La/Nb (<1) and Ba/Nb (<5) which is the characteristics of mantle plume derived melts. Trace element data of the Erta’Ale basalt show that the source of the Erta‘Ale basalt is Afar Mantle plume. The La/Nb, La/Sm and Ce/Pb versus time diagram indicate that the source, depth and degree of melting of the Erta’Ale basalt don’t have a significant change with time. The geochemical results show that there is no significant compositional variation in the Erta’Ale basaltic rock. However, there is a slight chemical variation between the 2017, 2010 and the others older lava. This variation is a result of the difference in degree of fractionation.Item Geochemical Investigation of the Bimodal Volcanic Suite at the Western Afar Margin: the Case of Shewa Robit Marginal Graben, Northern Shewa, Central Ethiopia(Addis Ababa University, 2020-12-01) Mulu, Kiflom; Ayalew, Dereje (Professor)The study area, Shewa Robit, is situated in the southern end section of the Western Afar Margin. It is located approximately 222 Km northeast of Addis Ababa. In this study, field, petrological and geochemical (major and trace elements) data of the volcanic rocks from the Shewa Robit area are presented and integrated to understand their petrogenesis. The main lithological units of the study area are lower basalt, glassy rhyolite, porphyritic rhyolite, rhyolitic ignimbrite, unwelded tuff and upper basalt. The petrographic study shows that the mafic volcanic products are characterized by aphyric to porphyritic textures with the plagioclase dominant phenocryst, whereas the felsic rocks are dominated by quartz and K-feldspar (sanidine) phenocrysts and glassy groundmass. The geochemical analysis results indicate that the Shewa Robit volcanic rocks are bimodal in composition with a significant compositional gap (between 52.75 and 69.8 wt. % SiO2) between the mafic and felsic members. It comprises a rock type, including basalts, trachy-basalts (Hawaiite), basaltic-andesite and rhyolites. The mafic volcanic rocks are olivine to quartz tholeiitic whereas the silicic rocks are metaluminous and peralkaline rhyolites. Trace element ratios of the mafic samples show some crustal contamination on samples from the lower basalts. Moreover, the investigated mafic samples from the study area exhibit trace element ratios such as Nb/U (36- 48.3), Zr/Nb (6-9.69), Ba/La (11.39-18.31), La/Nb (0.93-1.13) and Ba/Nb (12.3-20.7) and suggest that these lavas are derived from mantle source similar to OIB-type sources. Furthermore, mafic rocks of the volcanic suite are characterized by low CaO/Al2O3 ratios (0.47-0.72), enrichments at Ba, depletions at K and flat HREE patterns (TbN/YbN = 1.55-2.08). These major and trace element data indicate that basalts from the Shewa Robit area were derived from the melting of amphibole-bearing spinel peridotite source. Moreover, mafic and felsic rocks of the studied area are proved to be genetically related by the fractional crystallization process. The variations in major and trace element concentrations and petrographic studies indicate that fractionation of olivine, clinopyroxene, plagioclase, K-feldspar, apatite and Fe-Ti oxide or opaque minerals occurred during the evolution of the Shewa Robit bimodal basalt - rhyolite volcanic suite.Item Geochemical Studies of Volcanic Rocks for the Implication of their Petrogenesis in Between Southern Lake Hayk and Western Part of Lake Hardibo South Wollo Amhara Regional State Ethiopia(Addis Ababa University, 2021-09-14) Berlie, Getie; Ayalew, Dereje (Professor)The study area is located around Lake Hayk northeastern central part of Ethiopian. In this study petrographic, and geochemical (trace and major elements) data of mafic to felsic rock are presented, integrated, and interpreted to characterize the geochemistry of cogenetic suite and petrogenetic processes involved in the evolution of these flood basalts to differentiated rhyolite rock units. Fieldwork for sampling and mapping, petrographic investigations, and major and trace element geochemical analysis using ICP-AES and ICP-MS procedures are all completed to meet this objective. The area constitutes of bimodal composition of flood basaltic to rhyolitic rock with scarce intermediate composition. The rock units that make up the research area are basalt, basaltic andesite, trachydacite, and rhyolite, based on total alkalis versus silica (TAS) classification. The basaltic rock (La/Yb) N ratio = (5.34-10.97), basaltic andesite to rhyolite (La/Yb) N ratio = (6.19-11.74), basalt is less to moderately fractionated than moderately fractionated basaltic andesite to rhyolite. The basalt, basaltic andesite to rhyolite samples have ratios of Ce/Pb >19.65-33.2 and 6.13-15.6 respectively and basalt sample has ratios of Nb/U = 39.55- 45.81 and basaltic andesite to rhyolite have Nb/U= 21.59-33.68 and basalt and basaltic andesite to rhyolite has ratios of Nb/Th=9.7-12.3, 5.08-8.63 respectively. Basalt have low Rb/Nb = 0.5-0.58, La/Nb = 0.88-1.06 and high TiO2 = 2.08-3.04,basaltic andecite have higher Rb/Nb =2.7, La/Nb = 1.81 and low TiO2 1.96 and rhyolite Rb/Nb = 0.97-1.69, La/Nb = 0.51-1.08 and lower TiO2 = 0.41- 0.71.MgO against incompatible trace elements such as Th, La, and U has a negative correlation this testifies that there is insignificant continental crust involvement and Ce/Pb displays a positive correlation with MgO suggest that there is minimal crustal contamination. The positive and negative anomalies of Ba and K respectively, testify related to amphibole mantle source. The basaltic rock unit characterized by low CaO/Al2O3 ratios (0.71–0.99) and relatively no fractionation and flat HREE patterns with (TbN/YbN = 1.75-2.33) chondritic values this suggest a mantle source mostly containing spinel rather than garnet and the gentle slope in HREE show the source for mafic rock is garnet free. Rhyolite is characterized by a steep negative correlation in bivariate major oxides such as MgO,Fe2O3,TiO, and CaO against SiO2 and positive anomaly of Ta with slight Nb trough which implicate that Rhyolites formed by fractional crystallization rather than significant crustal contribution, partial melting and assimilation. Detail isotope geochemistry and mineral chemistry studies are much recommended around Lake Hayk.Item Geological and Geochemical Studies of Flood Basalts (Aiba and Ashenge Basalts) in Maychew Area, Tigray, Northern Ethiopia: Implication for their Petrogenesis.(Addis Ababa University, 2018-06-27) Mengesha, Hayelom; Ayalew, Dereje (Professor)The study area, Maychew, is located in the northern eastern corner of the northwestern Ethiopian plateau. Petrological, petrographic, and geochemical (trace and major elements) data are presented and integrated for Maychew flood basalts to characterize the geochemistry and constrain petrogenetic processes involved in the evolution of these flood basalts. According to these data two types of basaltic groups have been identified; tilted basaltic units (Ashenge basalts) and horizontal to sub horizontal basaltic units (Aiba basalts).The studied samples have low Mg numbers (Mg#:37.99-58.92, excluding sample T2S8 and T4O18) and low compatible element contents such as Ni and Cr suggesting that these lavas have undergone fractionation en route to the surface. Moreover Ce/Pb, La/Nb, and La/Ta ratios, which are sensitive to crustal contamination, of the studied samples indicates that crustal contamination is more pronounced on samples of Aiba basalt than Ashenge basalts. Further, the investigated samples from study area exhibit trace element ratios such as Zr/Nb (3.77-10.88), Ba/La (5.75-13.55), La/Nb (0.76-1.89) and Ba/Nb (4.65-18.07), which overlap substantially the field of OIB. This suggests that these lavas are derived from mantle sources that are similar to OIB-type sources or contain high proportion of OIB-type sources at least on the basis of trace element ratios. Moreover, a plot of two highly incompatible trace elements, thought to be insensitive to fractional crystallization and partial melting, indicates that both of the basaltic groups were originated from a common mantle source. However, the observed distinct trends of these basaltic groups on such plots, and on some major and trace element variation diagrams could be related to variation in degree of crustal contamination and/ or fractional crystallization, rather than source heterogeneity. Hence, the two basaltic groups are originated from a common OIB like mantle source following different paths of petrogenetic processes; crustal contamination and/or fractional crystallization.Item Magmatic Evolution of Boko Magmatic Complex, Main Ethiopian Rift, Using Petro-Chemical Approach and Co2 Flux Measurement(Addis Ababa Universty, 2016-07) Zafu, Amdemichael; Ayalew, Dereje (Professor)Boko magmatic complex is situated in transitional zone between Central and Northern MER segments. The area is located approximately 92.02 Km from Addis Ababa. The main objective of the research study is to understand the magmatic evolution and the extent of the main collapse zone of the complex. To meet the objectives, starting from the beginning different methods have been applied. The methods are; remote sensing, field observation, petrography, structural data analysis, geochemistry and soil CO2 degassing. The Boko magmatic complex is characterized by two main phase of eruption; Boko caldera forming eruptive activity and post caldera eruptive activity. These episodes give volcanic products that constitute the complex. The major volcanic units are rhyolitic lava flow, pumice flow, ignimbrite, lower pumice fall, rhyolitic lava dome, obsidian, lower basalt, upper ash flow, scoria and upper basalt. The surface thermal manifestation and anomalous CO2 degassing suggests Boko has currently active volcanic activity beneath the volcano. The volcanic products are affected by extensional tectonics which is manifested by appearance of geological structures like, joint and NNE-SSW trending major normal faults. The geochemical analysis results indicates the Boko rocks are bimodal in composition, mafic and silicic, lacking intermediate composition. The mafic rocks are transitional to weakly subalkaline basalt and the silicic rocks are dominantly peralkaline rhyolites. The two groups of rocks are co-genetic and related to each other by fractional crystallization process starting from mantle-derived basaltic magma with slight crustal material involvement. The soil CO2 degassing measurement result shows that it has biogenic and volcanic-hydrothermal source. The total flux from the two sources is 74.71 t d-1. The mean of CO2 flux from the biogenic source is 0.041 g m-2 d-1 and from volcanic-hydrothermal origin is 2.14 g m-2 d-1. The combination of remote sensing, soil CO2 flux value distribution, field observations and structural data analysis suggest the main collapse zone of Boko Caldera has elliptical geometry with a dimension of 6 Km (E-W) by 4 Km. The major caldera axis is along E-W which is in agreement with the Main Ethiopian Rift regional stress direction. Key words: Boko Magmatic Complex, Fractional crystallization, Crustal contamination, CO2 degassing, Caldera forming eruption, Caldera collapse and Main collapse areaItem Petrogenesis and Source Rock Characterization of Volcanic Rocks from the Gidole Horst in the Southern Ethiopian Rift(Addis Ababa University, 2021-09-13) Hangibayna, Asmamaw; Ayalew, Dereje (Professor)The eastern branch of East African rift system consists of Ethiopian and Kenyan rift systems. Ethiopian rift system start from the afar triple junction to the broaden zones of the over lapping rift systems segmented into three parts the northern, the southern and the central rift system. Gidole horst lies in the complex zone of the overlapping Ethiopian and Kenyan rift in the southern part of Ethiopia. It is bounded between Chamo basin to the East and Woyto basin to the west nearly orient in southeast-northwest direction. It has step up normal faults rise to an elevation 2540m above mean sea level. To constrain the petrogenetic evolution and source rock characterization of the Gidole horst volcanic rocks; integrated field, petrographic analysis and major and trace element geochemistry has been conducted. Thus, the study constrained petrogenesis of volcanic rocks and their association through petrographic, major and trace element data, and the source rocks of the basalts and the rhyolites through trace element models. The possible contamination of the basalts and the rhyolites were also addressed. The stratigraphic studies in the Gidole horst allowed the recognition of several phase of volcanic activity. The lowest exposed unit is the lower basalt equivalent to the Amaro horst lying over the metamorphic basement exposed around Gato. This unit has several phases of eruption and is overlain by felsic tuff. Felsic tuff consists of the lower ignimbrite and the upper unwelded tuff. It is sandwiched between the lower basalt and the middle basalts, where the middle basalts are characterized by columnar joints. A thin trachytic flow over lays the middle basalts around Gebele-Beno, specifically in Himbro expected to be the volcanic vent. The existence of mud stone overlaying these units indicates the presence of hiatus between the trachytic flow and the upper basalts. The upper basalts cover a large volume and the upper flows fall in the sub-tropical climate condition that are highly altered. Petrographically the rocks are classified as olivine-plagioclase phyric, aphyric, trachytic and felsic tuff. The very early flows of lower basalts show olivine-plagioclase phenocrysts with microphenocrysts of olivine, plagioclase and pyroxene. The groundmass of it consists of opaque’s in addition to the above minerals. Whereas, the upper flows of the lower basalt, the middle basalts are aphyric consists olivine, plagioclase, pyroxene and opaque minerals with less amphibole observed in the upper basalts. The trachyte show trachytic texture with plagioclase and plagioclase is absent in felsic tuff, instead large crystals of sanidine are observed. Major and trace element variation of Gidole horst volcanic rocks show strong association of the rocks and support the origin of the evolved rocks by crystal fractionation starting from the lower basalt to the more evolved products. The separation of olivine, plagioclase, pyroxene and opaque occurred in the basaltic rocks and addition of sodic plagioclase in the intermediate trachyte. Furthermore, the evolution generated strong enrichment of incompatible elements and depletion in compatible elements during the course of the fractionation. As a result the more evolved rhyolite have higher concentration of incompatible elements (e.g. Zr, Rb, Th, Ta and REE) and lower concentrations of compatible elements (Ni, Cr, Sr and Ba). As Ba and Sr substitute Ca in plagioclase, they are readily compatible to the early formed mineral, which is plagioclase. Consequently, Ba and Sr formed peak in the mantle normalized diagrams specifically in the lower basalts. Whereas the evolved produces, produce a trough for those elements. In addition to this, the rhyolites have a trough for P and Ti indicating the fractionation of apatite and Fe-Ti oxide. The strong correlation between the volcanic rocks shown on the petrographic analysis, major and trace element variation diagrams, the volcanic rocks of Gidole horst have shared comment source rather than displaying different sources. Thus, the lower basalt is the oldest and primitive basalt from which the other basalts and felsic rocks are evolved. Based on the strong fractionation of REE the model was introduced to determine the source of the rock and indicates they were derived by relatively small degrees of partial melting of a source in which, garnet remains as a residual. Even though they share common source there are some differences observed among the rocks. This is due to the contamination with rocks occurred during the rout to the surface below the upper crust.Item Petrogenesis of Bimodal Basalt-Rhyolite Suite from Debre Sahil/ Guna Area, Southeastern Ethiopia(Addis Ababa University, 2018-05-30) Wubishet, Tegene; Ayalew, Dereje (Professor)The Debre Sahil volcanic rocks are part of southeastern province, eastern shoulders of Northern MER. The area is located approximately 302 Km from Addis Ababa. The main objective of the research study was to understand the petrogenetic evolution of bimodal volcanic suite. To achieve the objectives, starting from the beginning different methods have been applied. The methods are; field observation, petrographic data analysis, and geochemical data analysis. The major volcanic rock units are pyroxene- olivine phyric basalt (upper basalt), olivine-pyroxene phyric basalt (lower basalt), white rhyolite, rhyolitic ignimbrite and glassy rhyolite. The petrographic study show that the mafic volcanic product of the area is characterized by olivine, clinopyroxene, plagioclase and Fe-Ti oxide crystal phases with microlite groundmass, whereas the rhyolitic rocks are dominated by alkali feldspar, plagioclase, quartz and lithic fragments with glassy groundmass. The geochemical analysis results indicate the Debre Sahil rocks are bimodal in composition, mafic and silicic, lacking intermediate composition. The mafic rocks are alkaline basalt and the silicic rocks are dominantly peralkaline rhyolites. Furthermore, these results indicate that the Debre Sahil volcanic rocks are sourced from spinel bearing peridotite with some residual garnet mantle source. Moreover, the basalt-rhyolite suite is proved to be genetically related to each other by fractional crystallization process. Selected trace element ratios such as Zr/Sm (25.3- 34.15), Zr/Hf (34.12-40.73), Nb/Ta (15.96-21.23), La/Nb (0.77-0.96), Nb/La (1.04-1.29) and Ba/Nb (10.27-21.67) indicates that the study samples are comparable with primitive mantle values, and thus are originated from enriched mantle source or OIB type source.Item Petrogenesis of Granitoid Rocks of Assosa Area, Western Ethiopia(Addis Ababa Universty, 2017-06) Wondera, Natnael; Ayalew, Dereje (Professor)The main objective of this research study to constrain the petrogenesis of granitoid rock of Assosa area. To achieve objective successfully, different research method had been done like: field investigation, petrographic analysis and description and geochemical data analysis and interpretation. Assosa granite rocks dominated by alkali feldspar, plagioclase feldspar and quartz mineral in addition to these the muscovite, biotite, sphene, and hornblend, garnet and opaque mineral occurred. Granitoid rock samples of the area characterized by medium to coarse-grained texture, euhedral to subhedral crystal shape, granular, intergranular texture of opaque mineral(Fe-Ti oxides) and alteration of plagioclase and alkali-feldspar lead to formation of sericite/muscovite and chlorite. An Assosa granitoid rock is categorized under alkali-feldspar granite and syenogranite of plutonic igneous rock classifications on QAP diagram. Scattered data follows and negative trend correlation of some major element versus to SiO2 indicate the occurrence of fractional crystallization and alteration or mobility elements. It has charactestics of both volcanic arc granitoid (VAG) using trace element tectonic discriminators like: Rb versus Y +Nb and Rb versus Yb + Ta and syn-collisional granitoid (Syn-COLG) Nb versus Y, which resembles subduction zone or island arc environment related granitoid rock formation. The Enrichments of LREE, negative Eu anomaly and slight to moderate flat HREE on Chondrite-normalized and highly deplement of Sr, Ti, P and negative anomaly of Nb and Ta multi variation element pattern diagram indicate common source region and arc-related magmas result from more mafic magmatic source that undergo fractional crystallization. High contents Uranium, Thorium and Potassium in both normalized and variation element diagram and negative anomalies of Nb and Ta indicate that granite of the area are mainly from continental crust of arc-related magmatism. Based on variation diagram of compatible (Sr) versus incompatible (Rb) on log scale representation wide variation in the concentration of Sr but the concentration Rb remain constant. This show that Assosa granite mainly resulted by fractional crystallization of mafic magma related to arc settingItem Petrogenesis of Plutonic Rocks from Southwest of Gimbi (Homa Area), Western Ethiopia(Addis Ababa Universty, 2017-05) Abule, Feye; Ayalew, Dereje (Professor)Southwest of Gimbi is located within the Western Ethiopian Precambrian shield that emplaced in between the low-grade volcanic and volcano-sedimentary rock assemblages and the high-grade gneiss and migmatites assemblages. The area is situated approximately about 508 km far from Addis Ababa. The main objective of the thesis is to characterize the petrology and geochemistry of plutonic rocks from Southwest of Gimbi area. To achieve the general and specific objectives of the research and come up with the expected result, starting from the beginning to the end different methods have been applied. Southwest of Gimbi Homa area is characterized by the plutonic rocks which have emplaced pre-, syn/late-to post-tectonic with associated to the major deformational events. Modally, plutonic rocks of Homa area range from rare diorite/gabbro through dominant granodiorite and monzogranite to alkali feldspar granite. Petrographic study shows that, Southwest of Gimbi Homa area granite rocks has medium to coarse grains and dominantly consist of K- feldspar phenocrysts, plagioclase, quartz and less biotite. All analyzed granite rocks from Southwest of Gimbi provide geochemical features of sub-alkaline series, high-K calc alkaline series, high silica concentration and high total alkali concentration. These granite rocks show enrichment in large ion lithophile elements and highly depleted in heavy rare earth elements. Southwest of Gimbi (Homa) granite rocks show enrichment in most incompatible elements (Rb and Pb) and depleted in compatible elements (Ba, Sr and Ti) reflecting the crustal source involvement in the rock genesis. Besides, the REE patterns of these granite rocks have subparallel and show pronounced negative Eu anomalies with increasing total REEs, typical of a feldspar fractionation trend. The considerable depletion in Nb, Ta and Ti and enrichment in large ion lithophile elements (Rb and Pb) on multi-element diagram indicates there is a subduction/volcanic arc related origin of granite rocks from Southwest of Gimbi. Southwest of Gimbi Homa granite rocks were genetically formed by small degree partial melting of crustal materials and by fractional crystallization of mantle derived mafic magma. Key words: Anomalies, Fractional crystallization, partial melting and ANS and MB.Item Petrologic and Petrographic Investigation of the Chewokare Volcanic Rocks, Southern Ethiopia(Addis Ababa University, 2018-05-30) Borku, Agegnehu; Ayalew, Dereje (Professor)Chewokare magmatic complex is situated in north of Lake Abaya NW continuity of Southern main Ethiopian rift with in Ganjuli graben bounded by post Hobicha and recent most felsic sources of Saluwa-Dere Hoko domes. The current studies focus on Petrology, petrography and geochemical aspect of Chewokare volcanic units. The Chewokare Volcanic rocks formed by two main phase of eruption; post Hobicha caldera forming eruptive activity and the most recent Saluwa Dere-Hoko felsic complex eruptive activity. The volcano tectonic evolution was started in early Miocene and which has been progressed to Plio-Pleistocene and most active rifting part is evident of Quaternary activity. Based on geochemistry the Chewokare volcanic rock classified in to alkaline to mildly subalkaline basalt, felsic products especially trachy andesite and trachy dacite. Major and trace elements geochemical data very important use to discriminate the type magmatic processes that involved during evolution of any type of volcanic product. Different major and trace data show there is two main processes determine the evolution of Chewokare volcanic rocks; fractionation and little crustal contamination. The diagrams of Nb-Zr and Hf-Th form a slightly positive correlation by forming a linear slope that show the genetic relationship or co-genetic nature of the units. Negative correlation of FeOt and MgO with SiO2; the first phase of crystallizing minerals are early formed one like Fe and Mg bearing mineral phases (eg, Olivine and Pyroxenes). The other phase that fractionate from the system is plagioclase feldspar that evidently indicated by the late dropping of CaO and Al2O3 trend line after some concentration of SiO2. The contamination index (La/Nb) ratio 2.17 this good indicator of contamination by crustal material and also the diagram La/Nb against MgO and Th/Ta or La/Nb against SiO2; which show inclined trend suggest acontamination. In general fractional crystallization involved with a minor crustal material involvement.Item Petrology of Flood Volcanism in Mertule Maryam, Northwestern Ethiopian Plateau, Central Ethiopia(Addis Ababa University, 2019-05-20) Fentie, Biniyam; Ayalew, Dereje (Professor)Mertule Maryam flood volcanism terrain is situated in the southern part of the northwestern Ethiopian plateau. The area is located 364 Km from Addis Ababa. The main objective of the thesis work is to understand the petrography and stratigraphy of Mertule Maryam area volcanic products and finally to assess the magmatic evolution. To accomplish the objectives, starting from the beginning various methods have been applied. The methods are; remote sensing, field observation, stratigraphy, petrography and geochemistry. The major volcanic units are plagioclase- pyroxene phyric basalt, agglomerate, aphyric basalt, plagioclase phyric basalt and undifferentiated aphyric basalt and tuff. The Mertule Maryam area volcanic products show narrow variation in composition of SiO2 percentage (47.7 – 51 wt %) as mafic suits. Petrographic and stratigraphic observation of mafic samples from the study area suggested three divisions within the study area. These are dominantly plagioclasepyroxene phyric, aphyric flow and plagioclase phyric flood basalt units. This distinctive petrographic heterogeneity reveals difference in depth of fractionation and magma flux in the lithosphere. The presence of plagioclase-pyroxene phyric basalt indicates there is considerable depth of fractionation in the shallow plumbing system. The aphyric basalt flow unit reflects there is no fractionation of any mineral in the shallow crust this indicates that the rate of magma flux to the shallow plumbing system is high. In addition the presence of plagioclase phyric basalt suggested that there was fractionation in the shallow plumbing system. Selected variation diagrams for major element and trace element shows scattered plot pattern which suggests these volcanic products are not the result of simple fractionation instead they do have different ways to come to the lithosphere.Item Petrostratigraphic Study of Volcanic Rocks in Sela Dingay Area North Shewa Central Ethiopia(Addis Ababa University, 2021-06-01) Sefa, Faysel; Ayalew, Dereje (Professor)A petrostratigraphic study of volcanic rocks in the parts of NW Ethiopian Plateau is presented in this paper. The study area is located in the North Shewa district, specifically around Sela Dingay, in central Ethiopia, about 200 kilometers north of Addis Ababa. The studied stratigraphic successions are cut by Mofar River starting from its tributaries in the highly elevated Termaber section (~3315m) downward into the lowly elevated Gawna section (1540m) in a way from Sela Dingay to Sasit in the central part of the study area. The main objective of the study was to provide a complete lithostratigraphy of the research area based on a thorough field investigation (mapping and description), stratigraphic sampling, and petrographic (thin section) analysis of representative samples. Furthermore, the study attempts to integrate these data to relate them with regional studies on geochemical, geochronological, and magma emplacement behavior data about the whole volcanic provinces of Ethiopia. From the composite stratigraphic section constructed in this study, five volcanic lithostratigraphic units (Lower Basalt, Sela Dingay Rhyolitic Lava flow, Upper Basalt, Sela Dingay Rhyolite-Ignimbrite, and Termaber Shield Basalt) which are a part of Cenozoic continental flood basalt, are identified. The volcanic rocks are underlain by Mesozoic (Cretaceous) Sandstone formation. The basaltic lava flows (fissural flows) are the dominant product with interlayered rhyolitic-pyroclastic deposits with the felsic products dominating the top part and the most top part of the succession is represented by central type shield basalt. These stratigraphic units are separated by a considerable time gap marked by alteration/erosional surface, paleosol, or sediments, which are all related to a decrease in magma flux in the system. Exposed flow fields display three-part features: flow top, flow core, and flow bottom structures. Flow tops are often made up of glassy to very fine-grained vesicular basalt, or scoriaceous to vesicular basalt fragments (brecciated flow top) that lie above a zone of non-fragmented, infrequently vesicular to columnar jointed basaltic core. The flow bottom is characterized by a limited zone of sparsely vesicular, glassy to very fine-grained basalt. A tabular to compound braided flow facies is confirmed by field observations of lava flow fields. A total of 24 petrographic samples from these stratigraphic units are studied by using a petrographic microscope. The majority of the samples are from Basaltic units and have a variety of textures ranging from aphyric to mildly phyric to porphyritic. Plagioclase and Olivine are the major phenocryst phase with a proportion of pyroxene and Fe-Ti-oxides. In some samples, the groundmass is made up of microlites of the same phase with a glassy texture. Cumulophyric, ophitic, and poikilitic to sieve textured phenocryst phases are embedded in a felty or trachytic groundmass in the porphyritic samples. In most basaltic samples, these features combine to generate porphyritic, holocrystalline to hypocrystalline, and Vitrophyric textures. Alkali feldspar, quartz, plagioclase, and lithic fragments with glassy groundmass predominate in rhyolitic and ignimbrite rock samples. The petrographic data from analyzed samples characterizes a part of HT1 flood basalt in geochemically zoned Ethiopian flood basalt. The predominance of plagioclase mineral as a phenocryst and groundmass phase and the occurrence of significant felsic-pyroclastic products coincides with the regional studies relating these character for the late-stage (termination phase) of flood basalt volcanism.Item Volcanic-Stratigraphy and Petrography of flood Volcanics Along Jita River in North-Central Ethiopia(Addis Ababa University, 2020-06-22) Belay, Endayen; Ayalew, Dereje (Professor)The study area is located in the North Wollo zone within the NW Ethiopian volcanic plateau. The area provides a well-preserved volcano-stratigraphic sequence from lower basalt initiation to upper lava flow termination. The main objective of this study is to establish the petrovolcanic stratigraphy of the area and then finally to put the petrographic observations within the context of magmatic evolution or depositional history of volcanic products. To accomplish the research objectives, actual field observation, stratigraphic sampling and petrographic analysis methods were applied. The major volcanic units from bottom to top are aphyric-intergranular basalt, aphyric-trachy flow basalt, augite cumulophyric basalt, olivine-augite phyric basalt, Kfs vitrophiric rhyolite, augite phyric basalt, basaltic tuff, columnar-aphyric basalt, non-columnar-aphyric basalt, moderately welded rhyolitic tuff, Kfs phyric rhyolitic-ignimbrite, Kfs phyric rhyolite, columnar-aphyric basalt, thin layer basaltic agglomerate and slightly vesicular aphyric basalt. Petrographyically, these volcanic rocks have different mineral compositions and textures; this heterogeneity reveals that there is a variation in depth of mineral fractionation and magma flux in the lithosphere. The presence of Cpx cumulophyric, Ol-Cpx and Cpx phyric basalt flows in the lower flow pile suggests that there is a considerable depth of fractionation in the deeper crustal level. The ol-megacryst composition and iddingsite alterations also reflect a constant replenishing of new primitive magma recharging the deeper plumbing system. Similarly, the presence of paleosoils, Kfs vitrophyric rhyolite and basaltic tuffs are also indications of cyclicity of magma eruptive phases in both shallow and deeper crust plumbing system with variable magma influx rate. However, the upper basaltic groups are extremely dominated with plagioclase microphenocrysts marked by agglomeratic deposit and capped with vesicular aphyric basalt. These observations indicate that the plumbing system was fed by shallow reservoir with decreasing rate of magma flux. Furthermore, the felsic units have dominant Kfs phenocrysts implying that the rate of magma flux was decreased and crystallized at shallow crustal level. The flows have broad changes in mineralogy and eruptive cyclicity and this suggests that they were pulsed with a fluctuating magmatic influx along with complex plumbing systems and overtime fed by shallower magmatic plumbing reservoirs. Overall, the petrovolcanic- stratigraphy findings in this study provide a new insight into the magmatic evolution of the Ethiopian CFB provinces.